As a technology of purifying exhaust gas of an internal combustion engine such as, for example, a diesel engine, there is known a technology of providing a diesel oxidant catalyst (DOC) or oxidation catalyst for oxidizing (i.e. burning) hydrocarbon (HC) and carbon monoxide (CO) in the exhaust gas, and a filter catalyst referred to as, for example, a diesel particulate filter (DPF) for collecting particulate matters (PM) composed mainly of carbon in the exhaust gas, or the like, in an exhaust passage of the internal combustion engine (refer to, for example, Patent documents 1 to 3).
If the amount of the particulate matters (hereinafter referred to as “PM” as occasion demands) collected in the filter catalyst increases, clogging of the filter catalyst may increase exhaust pressure and reduce the performance of the internal combustion engine. Thus, when the amount of PM collected in the filter catalyst reaches a certain amount, there is performed processing of supplying a reducing agent (e.g. fuel) to the filter catalyst and increasing the temperature of the filter catalyst to, for example, about 600 degrees C. so that PM is oxidized and removed (hereinafter referred to “PM regeneration processing as occasion demands) (refer to, for example, Patent documents 1 to 3).
In an exhaust gas purifying apparatus which is provided with the oxidization catalyst and the filter catalyst in this order from the upstream side of the exhaust passage, if the PM regeneration processing is performed by supplying the reducing agent into the exhaust gas from a reducing agent supplier which is disposed on the upstream side of the oxidization catalyst, the temperature of the oxidation catalyst increases because the supplied reducing agent reacts even with the oxidation catalyst (i.e. is oxidized) before reaching the filter catalyst. Moreover, in cases where such PM regeneration processing is performed, in order to increase the temperature of the filter temperature, for example, to about 600 degrees C., it is necessary to increase the temperature of the oxidation catalyst to a higher temperature (e.g. at about 700 degrees C.) than that of the filter catalyst, in view of thermal loss between the oxidization catalyst and the filter catalyst. This increases a thermal load on the oxidization catalyst which is disposed on the upstream side of the filter catalyst, and likely accelerates thermal degradation of the oxidization catalyst.
For example, the Patent document 1 discloses a technology of suppressing the thermal degradation of the oxidization catalyst by disposing the reducing agent supplier on the upstream side of the oxidation catalyst (first catalyst) and between the oxidization catalyst and the filter catalyst (second catalyst) to prevent the reducing agent from being wastefully supplied to the oxidization catalyst (first catalyst).
The Patent document 2 discloses a technology of reducing PM combustion temperature by mixing fuel added to exhaust with an additive containing ceria (cerium oxide, CeO) or the like. Moreover, the Patent document 3 discloses a technology of reducing an intake air amount in order to set a rich air-fuel ratio with a small amount of HC if the reducing agent (HC) is supplied by using nitrogen oxide (NOx) reduction or the like.